1. Field of the Invention
The present invention relates to temperature compensated piezoelectric oscillators of high spectral purity and capable of being controlled in frequency.
2. Description of the Prior Art
Prior art temperature compensated, controllable frequency piezoelectric oscillators consist of a temperature compensation circuit, a processing circuit, a first input of which takes the signal coming from the temperature compensating circuit and the second input of which takes a frequency control signal, a phase-shifting circuit for frequency control comprising the piezoelectric resonator followed by an oscillator circuit.
The frequency stability of a piezoelectric oscillator is a highly important parameter. In radio communications, the time bases should be as precise as possible, in the long term as well as in the short term. This means that the output frequency of the oscillator should vary as little as possible.
Ideally, a sinusoidal voltage is written as follows: EQU V(t)=Vo sin 2.pi.fo t
where Vo is the amplitude of the voltage, and fo is the frequency. However, in practice, this voltage becomes: EQU V(t)=(Vo+.epsilon.(t)) sin (2.pi.fo t+.DELTA.(t))
because of unwanted phenomena.
.epsilon.(t) represents the amplitude variation, PA1 .DELTA.(t) represents the phase variation or phase noise.
Since the phase and frequency are related, the frequency stability and phase noise are also related.
The phase noise can be measured by using a low frequency spectrum analyzer in the method where the phase of a reference oscillator is automatically locked into an oscillator to be tested. This phase noise decreases when the frequency in relation to the carrier increases.
An oscillator with high spectral purity will therefore have a phase noise, near the carrier, which is as low as possible.
In a piezoelectric oscillator, there are many parameters that cause frequency variations: these parameters are temperature, vibrations, shocks, acceleration, pressure, etc. Among these causes, temperature effects induce high frequency variation.
The thermally caused noise spectrum is found to a considerable extent in the low frequency phase noise spectrum.